The instant invention generally relates to occupant detection systems for controlling the activation of vehicular safety restraint systems and, more particularly, for determining the presence and position of an occupant for purposes of influencing the deployment of a safety restraint system responsive to a crash.
A vehicle may contain automatic safety restraint actuators that are activated responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such automatic safety restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters. One objective of an automatic restraint system is to mitigate occupant injury, thereby not causing more injury with the automatic restraint system than would be caused by the crash had the automatic restraint system not been activated. Generally, it is desirable to only activate automatic safety restraint actuators when needed to mitigate injury because of the expense of replacing the associated components of the safety restraint system, and because of the potential for such activations to harm occupants. This is particularly true of air bag restraint systems, wherein occupants too close to the air bag at the time of deploymentxe2x80x94i.e. out-of-position occupantsxe2x80x94are vulnerable to injury or death from the deploying air bag even when the associated vehicle crash is relatively mild. For example, unbelted occupants subjected to severe pre-impact braking are particularly vulnerable to being out-of-position at the time of deployment. Moreover, occupants who are of small stature or with weak constitution, such as children, small adults or people with frail bones are particularly vulnerable to injury induced by the air bag inflator. Furthermore, infants properly secured in a normally positioned rear facing infant seat (RFIS) in proximity to a front seat passenger-side air bag are also vulnerable to injury or death from the deploying air bag because of the close proximity of the infant seat""s rear surface to the air bag inflator module.
Yet another technique for mitigating injury to occupants by the air bag inflator is to control the activation of the inflator responsive to the presence and position of the occupant, thereby activating the inflator only when an occupant is positioned outside the associated at-risk zone of the inflator. NHTSA data suggests that severe injuries due to close proximity with the inflator can be reduced or eliminated if the air bag is disabled when the occupant is closer than approximately 4to 10inches from the inflator door. Such a system for disabling the air bag inflator requires an occupant sensor that is sufficiently sensitive and robust to make such a determination, while not causing the air bag inflator to be disabled when otherwise required for providing occupant restraint.
Except for some cases of oblique or side-impact crashes, it is generally desirable to not activate an automatic safety restraint actuator if an associated occupant is not present because of the otherwise unnecessary costs and inconveniences associated with the replacement of a deployed air bag inflation system. The prior art teaches various means for detecting the presence of an occupant, or the recognition of an inanimate object in the passenger-seat of a vehicle for purposes of implementing such a system. For example, weight sensors can be incorporated into the seat to detect the presence of an occupant.
Yet another technique for mitigating injury to occupants by the air bag inflator is to control the inflation rate or inflation capacity of the air bag inflator responsive to presence and position of an occupant. Such a control system would most preferentially be used in conjunction with a controllable inflation system responsive to crash severity, such as described above, wherein the occupant position inputs can be used to override otherwise overly aggressive air bag inflator controls which might otherwise be indicated by the particular crash severity level but which could be injurious to occupants of small stature or weight, or to infants in rear facing infant seats. Such a system for controlling the air bag inflator requires an occupant position sensor that is robust and sufficiently accurate, and that can distinguish and discriminate various occupant seating configurations and conditions. U.S. Pat. No. 5,071,160 and 5,118,134 teach the combination of sensing occupant position and/or velocity, and vehicle acceleration for purposes of controlling an inflator. Both of these patents teach by example the use of ultrasonic ranging to sense occupant position. U.S. Pat. No. 5,071,160 also teaches by example the use of a passive infrared occupant position sensor, while U.S. Pat. No. 5,118,134 teaches the use of a microwave sensor. U.S. Pat. No. 5,398,185 teaches the use of a plurality of occupant position sensors in a system for controlling safety restraint actuators in response thereto.
The prior art teaches the use of one or more ultrasonic beams reflected off the surface of an object to sense the location of the surface of the object. U.S. Pat. No. 5,330,226 teaches the combination of an ultrasonic ranging sensor mounted in the instrument panel and an overhead passive infrared sensor to sense occupant position for controlling a multi-stage air bag inflator or a vent valve connected thereto. U.S. Pat. Nos. 5,413,378, 5,439,249, and 5,626,359 teach ultrasonic sensors mounted in the dash and seat in combination with other seat sensors to detect the position and weight of the occupant for purposes of controlling an air bag inflator module. U.S. Pat. No. 5,482,314 teaches the combination of ultrasonic and passive infrared sensors together with associated signal processing for purposes of determining whether or not to deactivate a passive restraint system.
The prior art also teaches the use of infrared beams reflected off the surface of an object to sense the location of the surface of the object. U.S. Pat. No. 5,446,661, and 5,490,069 teach an infrared beam directed by a transmitter at a point of reflection on the object. A receiver detects the radiation scattered from the point of reflection, and measures the distance of the point of reflection from the transmitter based upon a triangulation of the transmitted and received beams for purposes of controlling the activation of a safety restraint system. These patents also teach the combination of an infrared beam occupant position sensor with an acceleration sensor for purposes of controlling an air bag inflation system. U.S. Pat. No. 5,549,323 teaches the incorporation of a light beam occupant sensor into an air bag door. Furthermore, infrared beam sensors are commonly used as rangefinders in automatic focusing cameras.
The prior art of U.S. Pat. Nos. 4,625,329, 5,528,698, and 5,531,472 teach the use of imaging systems to detect occupant position, the later two of which use this information for purposes of controlling an air bag inflator. U.S. Pat. Nos. 5,528,698, 5,454,591, 5,515,933, 5,570,903, and 5,618,056 teach various means of detecting the presence of a rear facing infant seat for purposes of disabling an associated air bag inflator.
The prior art also teaches the use of capacitive sensing to detect the presence, proximity, or position of an occupant. U.S. Pat. No. 3,740,567 teaches the use of electrodes incorporated into the base and back of the seat respectively, together with a capacitance responsive circuit, for purposes of discriminating between human occupants and animals or packages resting on an automobile seat. U.S. Pat. No. 3,898,472 teaches an occupant detection apparatus which includes a metallic electrode which is disposed to cooperate with the body of an automobile to form an occupant sensing capacitor, together with related circuitry which senses variations in the associated capacitance responsive to the presence of an occupant. U.S. Pat. No. 4,300,116 teaches the use of a capacitive sensor to detect people proximate the exterior of a vehicle. U.S. Pat. No. 4,796,013 teaches a capacitive occupancy detector wherein the capacitance is sensed between the base of the seat and the roof of the vehicle. U.S. Pat. No. 4,831,279 teaches a capacitance responsive control circuit for detecting transient capacitive changes related to the presence of a person. U.S. Pat. No. 4,980,519 and 5,214,388 teach the use of an array of capacitive sensors for detecting the proximity of an object. U.S. Pat. No. 5,247,261 teaches the use of an electric field responsive sensor to measure the position of a point with respect to at least one axis. U.S. Pat. No. 5,411,289 teaches the use of a capacitive sensor incorporated into the back rest of the seat to detect occupant presence. U.S. Pat. No. 5,525,843 teaches the use of electrodes incorporated into the base and back of the seat for purpose of detecting the presence of an occupant, whereby the electrodes are substantially insulated from the vehicle chassis when the detection circuit is active. U.S. Pat. No. 5,602,734 and 5,802,479 teach an array of electrodes mounted above the occupant for purposes of sensing occupant position based upon the influence of the occupant on the capacitance among the electrodes. U.S. Pat. No. 5,166,679 teaches a capacitive proximity sensor with a reflector driven at the same voltage as the sensing element to modify the sensing characteristic of the sensor. U.S. Pat. No. 5,770,997 teaches a capacitive vehicle occupant position sensing system wherein the sensor generates a reflected electric field for generating an output signal indicative of the presence of an object. U.S. Pat. Nos. 3,943,376, 3,898,472, 5,722,686, and 5,724,024 also teach capacitive-based systems for sensing occupants in motor vehicles.
In addition to methods taught by the above referenced U.S. Patents, the prior art also teaches various means of measuring capacitance, as for example given in the Standard Handbook for Electrical Engineers 12th edition, D.G. Fink and H. W. Beaty editors, McGraw Hill, 1987, pp. 3-57 through 3-65 or in Reference Data for Engineers: Radio, Electronics, Computer, and Communications 7th edition, E.C. Jordon editor in chief, Howard W. Sams, 1985, pp. 12-3 through 12-12, both included herein by reference.
The technical paper xe2x80x9cField mice: Extracting hand geometry from electric field measurementsxe2x80x9d by J. R. Smith, published in IBM Systems Journal, Vol. 35, Nos. 3and4, 1996, pp. 587-608, incorporated herein by reference, describes the concept of Electric Field Sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. What has commonly been referred to as capacitive sensing actually comprises the distinct mechanisms of what the author refers to as xe2x80x9cloading modexe2x80x9d, xe2x80x9cshunt modexe2x80x9d, and xe2x80x9ctransmit modexe2x80x9d which correspond to various possible electric current pathways. In the shunt mode, a voltage oscillating at low frequency is applied to a transmit electrode, and the displacement current induced at a receive electrode is measured with a current amplifier, whereby the displacement current may be modified by the body being sensed. In the xe2x80x9cloading modexe2x80x9d, the object to be sensed modifies the capacitance of a transmit electrode relative to ground. In the transmit mode, the transmit electrode is put in contact with the user""s body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
In one embodiment, a plurality of capacitive sensors are used to sense distances to the occupant, which in combination with the known locations of the fixed sensor elements are triangulated to locate the position of the occupant. One problem with such capacitive sensor arrangements is that they make use of the dielectric constant of known stability to detect the distance between a sensor and the occupant. Furthermore, the occupant position measurement tends to be associated with the center of mass of the sensed object. However, the sensor can be confused by large metal devices or arms/limbs in close proximity. Therefore, while these sensors may perform satisfactorily as an automatic xe2x80x9con/offxe2x80x9d switch to either disable the air bag inflator based upon occupant position, or enable the air bag inflator to be fired responsive to the activation signal from the vehicle crash sensor, the present embodiments of capacitive occupant position sensors may not be sufficiently accurate and robust to provide for controllable inflation based upon occupant position.
Occupant sensing systems that use capacitive sensors have significant problems when the sensor is wet and especially when the water near the sensor has good coupling to ground. The frequency dependent response of wet objects is discussed in an article describing capacitive sensing techniques by H. Philipp, entitiled xe2x80x9cThe Charge Transfer Sensorxe2x80x9d, from the November, 1996 issue of Sensors magazine, incorporated by reference herein. One prior-art capacitive sensing system that uses sensors in the seat back and the seat bottom reportedly has problems because the seat back angle creates changes in the sensor signals independent of the occupant situation.
Sensors which measure the distance between a point of reference and the surface of an object, such as ultrasonic or infrared beam sensors, are also vulnerable to false measurements, as would be caused for example by the presence of the extremities of an occupant, or by the presence of an object such as a scarf or newspaper held thereby, in proximity to the sensor. These types of sensors could be used to monitor the at-risk zone proximate the inflator door, but are subject to several disadvantages. In particular, infrared based systems usually incorporate a beam much narrower than the volume of the at-risk zone such that multiple beams may be required to reliably sense an object anywhere inside the at-risk zone. The incorporation of multiple beams results in extra cost, complexity, and potentially slowed response. Furthermore, both infrared beam and ultrasonic base sensors would require a significant amount of hardware proximate the inflator door if the at-risk zone proximate the inflator is to be monitored.
Some prior-art occupant detection systems attempt to identify the type of occupant or object in the passenger side seat, for example to discriminate a rear facing infant seat from a normally seated adult in the passenger seat. This is a very challenging task as there are a large variety of possible situations. Sensor systems that use distance measurements to identify occupant situations attempt to use information about relatively few points in space to identify the type of occupant in the seat from among many possibilities. Since the outer surface of any particular situation can change dramatically by doing something as simple as tossing a blanket over the occupant or changing the seat position, results are sometimes unreliable. Sensing systems that use some form of range sensing across significant distances within the occupant compartment can be blocked by objects such as newspapers, maps or floating balloons. Some occupant detection systems incorporate a complex algorithm that, while sometimes compensating for the lack of direct sensory information, can cause unpredictable or anomalous performance.
One disadvantage of many occupant detection systems is that they do not gather the most relevant information to determine if the occupant is in an at-risk zone around the inflator module. Occupant detection systems that are mounted above the passenger and look down on the seat area have the wrong physical perspective to directly monitor the region around the inflator door. Even if an ideal set of roof mounted sensors can reliably determine the occupant""s gross position xe2x80x94which is a very challenging task, xe2x80x94the actual volume between the inflator door and the occupant may be blocked to the sensors by the occupant""s body. If the criteria for controlling the activation of an air bag inflator were in part based on the proximity of the occupant""s body to the air bag inflator door, then overhead sensors simply cannot reliably obtain the relevant information. Systems that only use ultrasonic and optical sensing mechanisms can be blocked by newspapers. Ultrasonic sensors in some configurations will be affected by environmental conditions (temperature, humidity, altitude) because the speed of sound changes depending on the environment. Any sensing system that needs a clear line of sight between the sensor and the occupant requires the sensor to be visible to the occupant.
NHTSA recommends the use of towels under child seats to make them stable. Some prior-art sensing systems discriminate between child seats and occupants seated directly on the seat by their corresponding pressure patterns. A towel, or other object, placed under a child seat could make the child seat""s pressure pattern appear like an occupant seated directly on the seat, but would have relatively little effect on the electric field sensor of the capacitive sensing subsystem.
Another problem with some prior-art occupant detection systems is their inability to disable the air bag during a pre-impact breaking event.
The instant invention overcomes the above-noted problems by providing an occupant detection system comprising an electric field sensor and a range/proximity sensor. The electric field sensor comprises at least one electrode mountable in a seat bottom of a vehicle seat. The range proximity sensor comprises either a ranging sensor or a proximity sensor that senses the presence of an occupant proximate to the seat back of a vehicle seat. A sensing circuit operatively coupled to the at least one electrode of the electric field sensor generates a signal responsive to an electric-field-influencing property of an object on the vehicle seat. A controller operatively coupled to the electric field sensor and to the range/proximity sensor discriminates the type of object on the vehicle seat from the signal from the electric field sensor, and controls the activation of the safety restraint system responsive to the type of object, and responsive to a signal from the range/proximity sensor in the seat back indicating an occupant""s torso proximate to the seat back. The controller disables the restraint system if a normally seated occupant is not detected on the vehicle seat.
Accordingly, one object of the instant invention is to provide an occupant detection system that can discriminate normally seated occupants from other seat occupancy conditions.
A further object of the instant invention is to provide an occupant detection system that can disable a restraint actuator if a normally seated occupant is not present on a vehicle seat.
A yet further object of the instant invention is to provide an occupant detection system that is responsive to pre-impact braking conditions.
A yet further object of the instant invention is to provide an improved means of sensing the capacitance of an electrode in a vehicle seat of an electric field sensor for discriminating objects on a vehicle seat.
The electric field sensor in the seat bottom detects whether there is a large body immediately above the seat bottom cover as, for example, opposed to a child seat mounted on the passenger seat. The electric field sensor disables the air bag whenever no forward facing occupant is detected near the seat bottom, as would occur when any child seat (including RFIS, forward facing child seats and booster seats) is present on the seat, or when the seat is empty.
Accordingly, the electric field sensor provides a simple direct measure of whether there is a normally seated forward facing occupant in the front passenger seat. The electric field sensor in the seat bottom has a short range and only senses an occupant when a large surface of the occupant is very close to the sensor. Occupants normally seated directly on the seat cover always have a large surface of their body very close to the sensor. Infants or children in child seats have all, or most, of their body elevated several inches off the seat bottom surface which has relatively little effect on the sensor, whereby a failure to detect a normally seated occupant causes the air bag to be disabled. The electric field sensor senses characteristics of the normally seated occupant that are readily distinguished from a child in a child seat on the passenger seat. This method of sensing is highly advantageous in that the sensor signal is dependent on the dielectric characteristics of the passenger and does not simply sense the outer profile of the occupant in the same way optical or ultrasonic sensors do, which profile can change dramatically simply by throwing a blanket over the occupant or changing the seat position. This is even true of an empty seat situation. The dielectric characteristics proximate the seat bottom are relatively unaffected by changes in the profile of occupants and objects on the seat, such as caused by blankets. The sensor moves with the seat bottom so seat position or seat back angle do not affect the deployment decision.
Objects that are placed under child seats to stabilize the child seats don""t affect the deployment decision by the electric field sensor in the seat bottom, as can be the case for systems that incorporate seat weight sensors. A towel, or other object, placed under a child seat has relatively little effect on the electric field sensor of the electric field sensor in the seat bottom.
The electric field sensor is preferably implemented as a capacitive sensor, wherein the associated sensing circuit is adapted to measure the capacitance of at least one electrode of the sensor within the vehicle seat bottom. A plurality of electrodes may be used and separately measured so as to provide a measure of the distribution of an object on the vehicle seat bottom. The capacitance of the electrodes is relatively small, and the sensing circuit is adapted to provide calibrated capacitance measurements of the electrode by repeatedly comparing the measurement of the sensor electrode with measurements from one or more temperature stable reference capacitors. For example, a first reference capacitor is switched into the measurement circuit for a period of time. Then, an additional second capacitor is switched into the measurement circuit for an additional period of time, and the transient response to the combined capacitance is measured. Finally, the reference capacitors are switched out of the measurement circuit, and the at least one sensing electrode is switched into the measurement circuit so as to provide a measure of capacitance of the at least one sensing electrode. The sensing circuit is able to measure the absolute capacitance of the sensing electrode from this calibration incorporating two distinct and known reference capacitors in the measurement circuit. The sensing circuit is relatively robust and insensitive to temperature and temporal drift of the associated electronic componentsxe2x80x94excepting the reference capacitorsxe2x80x94because the sensing circuit is adapted to filter out D.C. offsets, and measurements are made during transients. More over, the sensing circuit incorporates a voltage follower and associated FET switches in a manner by which the capacitive elements that are not being measured can be effectively isolated from those which are being measured.
The electric field sensor can be adapted with additional electrodes, for example in the form of a driven shield, so as to reduce the influence upon the capacitance of the sensing electrode of liquids wetting the vehicle seat.
The instant invention also uses a range/proximity sensor in the seat back to determine if the occupant is seated against the seat back. Even if there is an occupant in an at-risk zone proximate to a restraint actuator, the restraint actuator is enabled the air bag if the occupant""s back is essentially against the seat back. This allows the air bag to deploy even in situations where the occupant""s hands are in the at-risk zone, but the occupant""s head or torso are safely outside the at-risk zone. And because the occupant cannot move with high velocities away from the seat back, the air bag can be enabled for a period of time after the occupant leaves the seat back. The range/proximity sensor in the seat back may comprise a second electric field sensorxe2x80x94particularly a capacitive sensor,xe2x80x94a radar sensor, an optical range sensor, an active infrared sensor, a passive infrared sensor, a vision sensor, an ultrasonic range sensor, or an inductive sensor.
Accordingly, the instant invention directly measures characteristics that are important for assessing whether the air bag deployment could be dangerous, i.e. if there is an occupant seated directly on the seat bottom, and whether the occupant is positioned to be at risk of injury by an inflating air bag. The air bag deployment decision is based on direct measurements and not on probabilistic predictions using indirect measures, resulting in more predictable and reliable performance. The instant invention disables the air bag for infants or children seated in infant or child seats on the passenger seat.
The instant invention provides a relatively simple systemxe2x80x94unaffected by seat position or seat back anglexe2x80x94for disabling the passenger air bag in nearly all situations where the air bag can be a hazard. The occupant""s head and torso need not be against the seat back for the system to accurately identify the occupant. Furthermore, objects that are placed under child seats to stabilize the child seats don""t affect the air bag inflator deployment decision.